What are the dimensions of the 20 gal? You would probably need a bigger tank once your bp gets bigger. That will probably be okay to start out, as long as the temperatures and humidity are okay. It should be around 90 on the hot side and 80 on the cool side. 70 degrees is definitely to low, even at night. It should not go below 75 degrees.
Humidity should be around 50 %-60%. You can measure it with a hygrometer. Walmart has great digital thermometer/hygrometer combos with a probe so that you can measure temps on both sides of the tank. I don't remember, but I think it's around $15. I also agree with Damien, sand would not be good for a ball python.

I'm glad you're getting your set up taken care of and doing research before you actually bring your ball python home. Keep asking questions.